1. Field of the Invention
The present invention relates to a power supply apparatus which transforms an input voltage and outputs a stable voltage and, more particularly, relates to a power supply apparatus which varies switching frequency that controls a switching element.
2. Description of the Related Art
Patent Document 1 showing a conventional example discloses one which reduces switching loss by fluctuating switching frequency to lower to the level of capable of switching during a high load period, and which can set the reactance value of a coil to an optimum value at the time of a low load and does not need to be set higher than necessary by returning the frequency to operate at stationary frequency during a low load period, by instructions from a microcomputer that controls the state of equipment.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2004-328834 (Paragraph 0017)
In the aforementioned conventional example, in the case where the load is constant, the switching frequency that controls a switching element is also constant. However, in automobile generators and the like, there is a case where an input voltage to be inputted to an input terminal is not constant even when the load is constant. In this case, for example, the switching frequency is set to be low in a state where the load is large; and therefore, in the case where the input voltage is high, a problem exists in that, when the switching frequency is low, a ripple current increases and exceeds an acceptable value. Incidentally, when the ripple current exceeds the acceptable value, an output smoothing capacitor generates heat due to internal resistance of the (output) smoothing capacitor and the deterioration of the output smoothing capacitor accelerates. When the deterioration of the output smoothing capacitor advances, there are harmful effects in that the output smoothing capacitor cannot absorb the ripple current, the output of a power supply apparatus is unstable, and equipment connected to the power supply apparatus erroneously operates; and such harmful effects have to be avoided.
This invention has been made to solve the problem described above, and an object of the present invention is to provide a power supply apparatus which varies switching frequency when an input voltage fluctuates and can set a ripple current to be not more than an acceptable value regardless of the input voltage.